Electronics > Metrology

A 100V battery bias supply for picoammeter measurements

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Gyro:
A while back I was trying to measure the leakage current of an electrostatic voltmeter with my low cost Picoammeter. I suddenly realised that I didn't have a sufficiently quiet low / no ripple bias supply for testing the leakage current of devices with any significant parasitic capacitance (or a capacitor), where ripple will swamp the Picoammeter input.

Not having the luxury an SMU, I don't know how other people handle this problem, but a reasonable voltage fully screened floating battery supply seemed like a simple answer. Our local Lidl also happened to have 230mAh CR2032 cells at less than £2 for a pack of six on the centre isle, which made the idea seem reasonably economical too. I settled on 100V as a suitable voltage - not that much help for a 1kV electrostatic voltmeter but probably adequate for most components that I might want to measure - 34 cells provde around 102V for the flat portion of the LiMnO2 discharge curve.

Construction proved not to be much of a problem. I picked up a short length of 21mm ID acrylic tube off ebay. The stack of cells is held under light spring pressure and a combination of metal and fibre washers (Lidl again). The two washers that actually make contact with the cell stack came from my junk box. I don't know what the plating on them is but it is readily solderable and shows continuity at the lightest touch of a blunt meter probe. The whole thing is enclosed in a piece of square section Ali extrusion with fabricated sheet ends and supported by O rings and foam. I included a polarity switch, the only sensible place to put one with a screened output.

I haven't built the test chamber yet, but I intend to put a 10 x 1M resistive divider inside to provide 10V steps. With the 10uA load of the resistor chain, the load life of the battery stack will be thousands of hours and, with practical usage, I should still achieve the expected shelf life of 10 - 15 years.

MiDi:
Another simply method is to stack 9V Batteries.
11 in series will give ~100V and cost ~15$ for ~500mAh Alkaline.
Shelf life is around half of CR2032 though, but otherwise they have better bang for the buck.

Gyro:
Yes, I did seriously think about 9V batteries, they're certainly much easier to clip together as needed. I wanted a flatter discharge curve than Alkaline though (and something a bit more compact than 11-12 batteries). At very low load, LiMnO2 give a virtually flat 3V for 80-90% of their shelf life.

DavidKo:
Lidl batteries has short shelf life, but who cares for that price. Since you have small current loading maybe you can combine different types of batteries and this can be even cheaper solution. There are 24 CR20xx batteries ;). Pity that they had stopped selling 9V accumulators (at least in our country).

Gyro:
Ha, yes. That would have made a more interesting battery stack, maybe I could have introduced different voltage taps with higher mAh rating on the lower voltages. I've never seen Lidl sell 9V rechargeables (I have looked too). They sell a NiMh charger for AA, AAA, and 9V but alas no 9V batteries to go with it. A shame because their other NiMh cells are really good.

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